Early systems and method for seismic well logging is disclosed in U.S. Pat. No. 3,093,810, re-issued as No. RE 25,928, to R. L. Geyer et al. The present invention is an improvement to the system disclosed by Geyer and can be incorporated in the system therein disclosed. Accordingly, the Geyer et al patent is specifically incorporated herewith by reference to supplement the following detailed description.
Useful information concerning prior art seismic log displays may also be found in U.S. Pat. No. 3,302,165 to Anderson et al.
As will be seen herein, the terms "seismic" and "acoustic" are used interchangeably with the same meaning.
In systems such as disclosed by Geyer et al, all the information of an acoustic wave is available to the viewer if the entire wave is displayed in an X-Y manner such as the usual oscilloscope display. Although this is sometimes done at relatively coarse vertical sampling intervals and at a depth scale presentation compatible with other well logs, it is not practical at a desired resolution in the order of, for example, one sample per inch of bore hole depth. If X-Y wave forms were made at this desired vertical resolution, it would require tens of thousands of displays for even a modest length of bore hole and, consequently, they could not be compared directly to other well logs run at normal depth scales.
The Geyer et al system overcomes some of the disadvantages of the X-Y presentation as described above. As may be seen, the Geyer et al system employs successive horizontal sweeps with no vertical deflection and with a trace intensity modulated according to wave amplitude, thus providing an X-Z form of display. Obviously, since little vertical display space is required, the Geyer et al type of display can be presented at desired depth resolution.
One disadvantage of the variable density X-Z display is that not all the wave information essential to some kinds of analysis is discernable.
Since various amplitudes are represented as various "shades of grey" in a manner of speaking, the large and average amplitude changes representative of fundamental single frequency, or of the average of combined frequencies, is readily seen.
However, the small anomalies, local to some portion of the principle gross wave, are not readily discernable, if at all. These are the anomalies that can indicate the beginning of an additional wave (such as an acoustic shear wave) which are very important in some analyses of acoustic logs.
Another disadvantage of the commercially available X-Z variable type display is that only the excursions in one polarity from a reference are presented, permitting the loss of local anomalies in the opposing polarity, even if such local anomalies were otherwise discernable.
The present invention provides a new type of display which, like the variable intensity display mentioned above, employs successive horizontal sweeps with no vertical deflection, permitting display at any desired depth resolution.
Unlike the earlier systems, the present invention does not employ variable intensity modulation. In contrast, the present invention employs turning the recording beam (or voltage) either on or off in what may be considered a dichotomous or bi-stable form of Z modulation. In this dichotomous Z mode of modulation, the recorded signals are indicative of the polarity of change of amplitude.
The term "polarity of change" may be thought of as another expression meaning the mathematical sign of the derivative of a signal. That is, when a signal is rising, thus having a positive derivative, the signal has a positive "polarity of change." Likewise, when the signal is falling, thus having a negative derivative, the signal has a negative "polarity of change." The "polarity of change" is thus a characteristic having two possible values, depending on whether the signal is increasing (becoming more positive) or decreasing (becoming more negative).
For example, the recording or "on" condition of a signal beam could commence each and every time the amplitude of the acoustic wave form changes from negative-going to positive-going and cease at the next transition from positive-going to negative-going. The intervening spaces of "off" condition of the beam would then continue to the next transition from negative-going to positive-going amplitude, thus beginning the next recording period.
In another embodiment the "recording" and "space" periods could just as well represent the opposite sense of signal amplitude change.
The detection of amplitude sense reversals may be readily detected by digitizing the wave form and computer-picking such reversals, as slightly delayed from real time, or alternately to make the same determinations in an analog manner. It will be seen that important local anomalies are clearly discernable by the display herein described.
Alternately, the recording period may begin with a reversal in the direction of amplitude change and end with the next reversal to the opposite polarity as previously described, but rather than being a constant maximum mark condition as previously described, the intensity during the recording period could be varied according to amplitude as is done in the earlier X-Z system shown by Geyer et al.
In a further modification of the present invention, a display system is provided that will indicate local anomalies regardless of whether a reversal of direction of amplitude change occurs or not. In such modification, the trend of the amplitude change is established and a next amplitude value is predicted by extrapolation with the predicted amplitude value compared to the actual value to establish a "variance" from the predicted amplitude value regardless of the sign of the variance. Such variance may then be employed by means of sensing variances in excess of a chosen "maximum permissible variance" and displaying a recording during periods during which the variance is in excess of the maximum permissible value.
This described maximum permissible variance is adjusted so that, during a single frequency wave or a wave of combined frequencies which are devoid of any distinct local anomalies, the maximum permissible variance would be exceeded only during the time or near the time that the reversals associated with the peak values of the wave are exceeded. The system herein described also indicates all local anomalies that create variances in excess of the maximum permissible variance.
In a preferred method of accomplishing the above described variance control display, the wave form is digitized and compared with previous and/or subsequent values in the wave form to create an arithmetic difference. When said difference has exceeded a preset maximum difference, then a recording is shown. Digital computers presently in commercial use in well logging systems can be programmed to accomplish the foregoing.